Field
[0001] The present disclosure relates to systems and methods for microwave photonics-enabled
beam-forming and channelization.
Background
[0002] In conventional radio frequency (RF) designs, phased array antenna beam forming relies
on electronically controlled phase shifters to align independent array elements. Both
the RF components and phase shift components create bandwidth and dynamic range limitations.
Increasing the bandwidth introduces RF beam perturbations such as beam squint. In
digital beam forming after initial down conversion to an intermediate frequency, a
high sampling rate analog-to-digital conversion (ADC) is needed, but this comes as
a tradeoff to ADC resolution. This limits the modulation order and ultimately reduces
the available data rate. Current systems for satellite, airborne and ground communications
are looking to increase capacity through bandwidth efficient modulation schemes (e.g.,
digital video broadcast satellite 2 ((DVBS-II)) that implement waveforms greater than
128 Quadrature Amplitude Modulation (QAM) and wider bandwidth channels in the millimeter
wave (mmW or MMW) spectral bands. This expansion in bandwidth and modulation order
creates challenges for the front end radio frequency (RF) and digital sampling components.
The same issues are present in transmission when all electrical channelization and
beam forming are used.
Summary
[0003] The following presents a simplified summary in order to provide a basic understanding
of some aspects of one or more implementations of the present teachings. This summary
is not an extensive overview, nor is it intended to identify key or critical elements
of the present teachings, nor to delineate the scope of the disclosure. Rather, its
primary purpose is merely to present one or more concepts in simplified form as a
prelude to the detailed description presented later.
[0004] In accordance with examples of the present disclosure, a receiver is disclosed. The
receiver comprises a phased array antenna having P receiving elements configured such
that each of the P receiving elements modulates one of P optical modulators; a first
optical frequency comb configured to generate a multitude of equidistantly spaced
optical wavelengths; a first 1 by P wavelength division multiplexer (WDM1) configured
to receive the multitude of equidistantly spaced optical wavelengths and further configured
to split the multitude of equidistantly spaced optical wavelengths into individual
optical wavelengths such that each individual optical wavelength is outputted as a
separate carrier signal to each of the P optical modulators such that a total incoming
radio frequency (RF) beam field incident on each of the P receiving elements modulates
a separate optical wavelength in each optical modulator; a second wavelength division
multiplexer (WDM2) configured to receive each output from each of the optical modulators,
such that all modulated wavelengths are combined together into a single output to
form a combined signal; and an N input ports by M output ports optical switch configured
to receive the combined signal from the WDM2 into at least one input port, and further
configured to switch the combined signal to an output port terminated in one Fiber-Bragg
grating (FBG), each FBG having different wavelength dispersion properties, such that
different time delays are introduced between different wavelengths in each fiber.
[0005] In some examples, in the receiver, the first optical frequency comb is configured
such that adjacent frequency separation is more than 2 times larger than a maximum
RF frequency expected to be received by phased array receiving elements.
[0006] In some examples, in the receiver, the first wavelength division multiplexer (WDM1)
has channel to channel separation equal to an adjacent frequency spacing of the first
optical frequency comb.
[0007] In some examples, in the receiver, the second wavelength division multiplexer (WDM2)
has channel to channel separation equal to an adjacent frequency spacing of the first
optical frequency comb.
[0008] In some examples, in the receiver, the second wavelength division multiplexer (WDM2)
is configured to have a band-pass more than 2 times larger than a maximum RF frequency
expected to be received by the phased array receiving elements.
[0009] In some examples, in the receiver, a number of channels in WDM1 and WDM2 are at least
a number of phased array receiving elements.
[0010] In some examples, in the receiver, optical paths between WDM1 and WDM2, including
each optical modulator are equal within 1mm difference.
[0011] In some examples, the receiver further comprises a first optical amplifier and a
first wavelength independent splitter with N output ports, wherein the first optical
amplifier and the first wavelength independent splitter are interposed between the
WDM2 and the optical switch such that the WDM2 outputs to the first optical amplifier
which in turn outputs to the first wavelength independent optical splitter which in
turn outputs to each input port of the optical switch.
[0012] In some examples, in the receiver, the FBGs at outputs of the optical switch are
configured to introduce time delays between the different wavelengths that are representative
of true time delays with which phased array receiving elements are illuminated from
RF beam field wave-fronts arriving from different directions.
[0013] In some examples, in the receiver, a number of N inputs for the optical switch corresponds
to a number of RF beams that can be simultaneously acquired on the phased array receiving
elements.
[0014] In some examples, in the receiver, a number of M outputs from the optical switch
correspond to a number of different RF beam directions that can be resolved.
[0015] In some examples, in the receiver, the optical switch is configured to enable selection
of the output port with the FBG that introduces specific true time delays between
wavelengths representative of a selected RF beam direction.
[0016] In some examples, in the receiver, the receiver further comprises a first optical
circulator that is interposed between the WDM2 and the FBGs.
[0017] In some examples, in the receiver, an output of the first optical circulator carries
all wavelengths representing signals from individual phased array elements forming
a specific beam direction due to a true time delay imposed between different wavelengths.
[0018] In some examples, in the receiver, the output of the first optical circulator is
routed to a signal channelization and processing elements.
[0019] In some examples, the receiver provides for simultaneous parallel acquisition processing
of N RF beam directions to occur.
[0020] In accordance with examples of the present disclosure, a transmitter is disclosed.
The transmitter comprises an optical frequency comb configured to generate a multitude
of equidistantly spaced optical wavelengths; an electro-optic modulator (EOM) configured
to receive the multitude of equidistantly spaced optical wavelengths and a data signal
and produce a modulated optical beam that is modulated with the data signal; an optical
circulator configured to receive the modulated optical beam at an input port; an N
input ports by M output ports optical switch configured to receive the modulated optical
beam from a first output port of the optical circulator, and further configured to
switch the modulated optical beam to an output port of the optical switch terminated
in one or more Fiber-Bragg gratings (FBGs), each FBG having different wavelength dispersion
properties, such that different time delays are introduced between different wavelengths
in each fiber; a 1 by P wavelength division multiplexer (WDM1) configured to receive
individual wavelengths of the modulated optical beam that are time-delayed from a
second output port of the optical circulator; and a plurality of antenna elements,
wherein each of the plurality of antenna elements configured to receive and transmit
an individual wavelength of the modulated optical beam.
[0021] In some examples, in the transmitter, each of the plurality of antenna elements comprise
a photodiode and a power amplifier.
[0022] In some examples, in the transmitter, the EOM is a Mach Zehnder modulator.
Brief Description of the Figures
[0023] The accompanying drawings, which are incorporated in, and constitute a part of this
specification, illustrate implementations of the present teachings and, together with
the description, serve to explain the principles of the disclosure.
FIG. 1 shows a receiver according to examples of the present disclosure.
FIG. 2 shows a system for channelization with two coherent frequency combs according
to examples of the present disclosure.
FIG. 3 shows a method of receiving according to examples of the present disclosure.
FIG. 4 shows a method of signal channelizing according to examples of the present
disclosure.
FIG. 5 shows a method of beam scanning and signal acquisition used in a search pattern
scenario according to examples of the present disclosure.
FIG. 6 shows a transmitter according to examples of the present disclosure.
[0024] It should be noted that some details of the figures have been simplified and are
drawn to facilitate understanding of the present teachings rather than to maintain
strict structural accuracy, detail, and scale.
Detailed Description
[0025] Reference will now be made in detail to exemplary implementations of the present
teachings, examples of which are illustrated in the accompanying drawings. Wherever
convenient, the same reference numbers will be used throughout the drawings to refer
to the same or like parts.
[0026] Generally speaking, a receiver and a transmitter are disclosed that are applicable
to space, air or ground RF communication systems and mitigate the common performance
parameter constraints (e.g. bandwidth and dynamic range) associated with a traditional
RF design. The disclosed approach is applicable to systems where one or more signals
of multiple types and characteristics are present in any given beam such as a communication
spot beam on a high-throughput satellite.
[0027] The receiver comprises a first optical frequency comb that is configured to generate
a multitude of equidistantly spaced optical wavelengths. The receiver also comprises
a first 1 by P wavelength division multiplexer (WDM1) that is configured to receive
the multitude of equidistantly spaced optical wavelengths and is further configured
to split the multitude of equidistantly spaced optical wavelengths into individual
optical wavelengths such that each individual optical wavelength is outputted as a
separate carrier signal to each of the P optical modulators such that a total incoming
radio frequency (RF) beam field incident on each of the P receiving elements modulates
a separate optical wavelength in each optical modulator. The receiver further comprises
a second wavelength division multiplexer (WDM2) that is configured to receive each
output from each of the optical modulators, such that all modulated wavelengths are
combined together into a single output to form a combined signal. The receiver still
further comprises an N input ports by M output ports optical switch that is configured
to receive the combined signal from the WDM2 into at least one input port, and further
configured to switch the combined signal to an output port terminated in one Fiber-Bragg
grating (FBG), each FBG having different wavelength dispersion properties, such that
different time delays are introduced between different wavelengths in each fiber.
For example, typical arrayed waveguide gratings (AWGs) used in telecommunication have
32 or 40 ports. In some instances, AWGs can have up to 128 ports. However, the performance
for higher count ports tend to begin to degrade. For typical commercial off the shelf
switches, N=96 and M=96 are available. N and M can be different combinations and for
custom assemblies the ports count may be increased with N<M, thus allowing larger
number of beam directions.
[0028] The transmitter comprises an optical frequency comb that is configured to generate
a multitude of equidistantly spaced optical wavelengths. The transmitter also comprises
an electro-optic modulator (EOM) that is configured to receive the multitude of equidistantly
spaced optical wavelengths and a data signal and produce a modulated optical beam
that is modulated with the data signal. The transmitter further comprises an optical
circulator that is configured to receive the modulated optical beam at an input port.
The transmitter still further comprises an N input ports by M output ports optical
switch that is configured to receive the modulated optical beam from a first output
port of the optical circulator, and further configured to switch the modulated optical
beam to an output port of the optical switch terminated in one or more Fiber-Bragg
gratings (FBGs), each FBG having different wavelength dispersion properties, such
that different time delays are introduced between different wavelengths in each fiber.
The transmitter still further comprises a 1 by P wavelength division multiplexer (WDM1)
that is configured to receive individual wavelengths of the modulated optical beam
that are time-delayed from a second output port of the optical circulator. The transmitter
still further comprises a plurality of antenna elements, wherein each of the plurality
of antenna elements configured to receive and transmit an individual wavelength of
the modulated optical beam. The transmitter and receiver as disclosed herein provides
for a number of improvements over the conventional devices. First, the use of the
optical frequency comb (also called an optical fiber comb) allows for the total RF
field that is incident on an antenna phase array to be modulated onto the optical
frequency comb with each wavelength carrying the field on one phased array element.
By using optical dispersion to introduce time delay between the different wavelengths,
which corresponds to a phase shift between the different RF elements, reconstruction
of one of the incident beams can be achieved independent of the frequency of the incident
RF field. By combining the reconstructed beam with another optical frequency comb
allows for channelizing down to smaller bandwidths. Combining an NxM switch with fiber
brag grading allows for forming a fast reconfigurable true time delay based RF beam
former. The transmitter and receiver are electronically switchable and allows for
fast beam scanning in search or tracking applications. The NxM switch and FBGs can
be implemented with several of the FBGs tunable. This will allow very fine beam pointing
control once the initial search has localized the beam. The NxM switch can be used
in a tracking mode, where a satellite as it passes above the Earth sequentially switches
a channel to different beam pointing angles, but simply routing to a different dispersion
FBG. The NxM switch with the FGB beam forming does not depend on the RF signal frequency
and bandwidth. Once the OFC frequency separation is correctly selected in order to
allow sufficient space for the modulated sidebands, the RF carrier frequency and the
total RF bandwidth have no effect on the beam forming.
[0029] FIG. 1 shows receiver 100 according to examples of the present disclosure. Receiver
100 comprises plurality of antenna phased array receiving elements 105 that are configured
to receive incident RFs 110 from a variant of angles. Each of plurality of phased
array receiving elements 105 comprise receiver Low Noise Amplifier (LNA) 115 and optical
modulator 120. Receiver 100 also comprises first optical frequency comb (OFC 1) 125
that is configured to generate a multitude of equidistantly spaced optical wavelengths.
In some examples, first OFC 1125 is configured such that adjacent frequency separation
is more than 2 times larger than a maximum RF frequency expected to be received by
phased array receiving elements. Other suitable frequency separations can be used.
[0030] Receiver 100 also comprises first 1 by P wavelength division multiplexer (WDM1) 130
that is configured to receive the multitude of equidistantly spaced optical wavelengths.
First WDM1 130 is further configured to split the multitude of equidistantly spaced
optical wavelengths into individual optical wavelengths such that each individual
optical wavelength is outputted as a separate carrier signal to each of the P optical
modulators 120 such that a total incoming radio frequency (RF) beam field incident
on each of the P receiving elements 105 modulates a separate optical wavelength in
each optical modulator 120. In some examples, first WDM1 130 has channel to channel
separation equal to an adjacent frequency spacing of the first optical frequency comb.
Other suitable channel-to-channel separation can be used.
[0031] Receiver 100 further comprises second wavelength division multiplexer (WDM2) 135
that is configured to receive each output from each of the optical modulators, such
that all modulated wavelengths are combined together into a single output to form
a combined signal. In some examples, second WDM2 135 has channel to channel separation
equal to an adjacent frequency spacing of the first frequency comb. Other suitable
channel-to-channel separation can be used. In some examples, second WDM2 135 is configured
to have a band-pass more than 2 times larger than a maximum RF frequency expected
to be received by the phased array elements. Other suitable band-pass parameters can
be used. In some examples, a number of channels in WDM1 130 and WDM2 135 are at least
a number of phased array receiving elements. Other suitable number of channels can
be used. In some examples, optical paths between WDM1 130 and WDM2 135, including
each optical modulator are equal within 1mm difference. Other suitable optical path
lengths can be used.
[0032] Receiver 100 also comprises first optical amplifier 140 that receives the output
of WDM2 135, and which provides an input to first wavelength independent optical splitter
145. First optical amplifier 140 and first wavelength independent splitter 145 are
interposed between WDM2 135 and optical switch 160 such that WDM2 135 outputs to first
optical amplifier 140, which in turn outputs to first wavelength independent optical
splitter 145, which in turn outputs to each input port of optical switch 160. Optical
switch 160 can be micro-electromechanical system (MEMS)-based or piezoelectric-based
non-blocking optical cross connect switches. Optical switch 160 connects any input
to any output and does not have a dependence on signal baud rate/RF modulation and
has typical signal loss of less than 1dB.
[0033] The output is provided to first optical circulator 150 and second optical circulator
155. First optical circulator 150 that is interposed between WDM2 135 and FBGs 165.
In some examples, second optical circulator 155 can be a Pth circulator, where P is
equal to the number of channels in WDM2 135. In some examples, an output of first
optical circulator 150 and/or second optical circulator 155 carries all wavelengths
representing signals from individual phased array elements forming a specific beam
direction due to a true time delay imposed between different wavelengths.
[0034] Receiver 100 still further comprises N input ports by M output ports optical switch
160 that is configured to receive the combined signal from WDM2 135 into at least
one input port. According to figure 1, the receiver 100 also comprises Fiber-Bragg
gratings (FBGs) 165.
[0035] Optical switch 160 is further configured to switch the combined signal to an output
port terminated in one Fiber-Bragg grating (FBG) 165, or another suitable fiber-based
wavelength dispersive element-based material, each FBG having different wavelength
dispersion properties, such that different time delays are introduced between different
wavelengths in each fiber. FBG 165 are structured (periodic or chirped) variations
of the index of refraction in a fiber. These index perturbations induce wavelength
dispersion or simply form wavelength selective reflecting mirrors. For example, FBG
165 comprises linearly chirped variations along the length of the fiber, which serve
as narrowband wavelength selective reflecting mirrors. The separation between the
different FBGs introduces time delay between different wavelengths. In some examples,
the FBGs at outputs of the optical switch introduce time delays between the different
wavelengths that are representative of true time delays with which phased array receiving
elements 105 are illuminated from RF beam field wave-fronts arriving from different
directions. In some examples, a number of N inputs for optical switch 160 corresponds
to a number of RF beams that can be simultaneously acquired on the phased array receiving
elements 105. In some examples, a number of M outputs from optical switch 160 correspond
to a number of different RF beam directions that can be resolved. In some examples,
optical switch 160 enables selection of the output port with the FBG that introduces
specific true time delays between wavelengths representative of a selected RF beam
direction.
[0036] Receiver 100 further comprises second optical frequency comb (OFC 2) 170. OFC 1125
generates optical wavelengths separated by 25 to 200 GHz and OFC 2 170 generates optical
wavelengths separated by 25 to 200 GHz plus a small offset ranging from 25 to 200
MHz. Output of OFC 2 170 is provided to second optical amplifier 175 and then to second
wavelength independent optical splitter 185.
[0037] According to figure 1, the receiver 100 also comprises signal channelization and
processing elements 190.
[0038] The output of optical circulator 135 and second wavelength independent optical splitter
185 are then routed to signal channelization and processing elements 190 and to digital
signal processor (DSP) 195 for processing. DSP 195 allows electronic amplitude balancing
between the signals acquired by the individual RF antennas. The true beam forming
involves aligning the phases with the true time delay, as described above, equalizing
amplitudes to compensate for possible systematic losses in the receiver. This can
be done electronically since it is easy to digitally change the magnitude of the signal.
Receiver 100 allows for simultaneous parallel acquisition processing of N RF beam
directions.
[0039] FIG. 2 shows system for channelization with two coherent frequency combs 200 according
to examples of the present disclosure. In order to down-convert and channelize the
acquired RF bandwidth, two coherent optical combs are used with relative frequency
offset Δ. The offset is selectable and can range between 100 KHz to 1 GHz. In the
example shown in FIG.2, the offset is fixed at 25 MHz. In this example, first frequency
comb 210 and second frequency comb 215 can have 200 lines separated by 25 GHz and
24.75 GHZ, respectively, occupying around 40 nm of gain spectrum. Frequency combs
composed of silica and silicon-based frequency combs have been demonstrated with greater
than 100 nm gain spectrum. The RF signal (data) is multicast on the 25 GHz frequency
comb using MZI. Second frequency comb 215 at 24.75 GHz serves as an optical Local
Oscillator (LO). Due to the frequency difference between first frequency comb 210
and second frequency comb 215, the LO slices the spectrum of the RF signal and each
slice is converted to I and Q streams by the coherent detectors.
[0040] As shown in FIG. 2, output of master laser 205 is split and provided to first tunable
optical frequency comb (TOC 1) 210 and second tunable optical frequency comb (TOC
2) 215. Spectrum 220 from TOC 1 shows a plurality of equally spaced optical frequencies.
Electro-optical modulator 225, such as a Mach-Zehnder modulator (MZM) receives data
230 and the output from TOC 1 210 and modulates data 230, resulting in modulated spectrum
235. Output of electro-optical modulator 225 is provided to first arrayed waveguide
grating (AWG 1) 240 and output of TOC 2 215, which functions as optical local oscillator
245, is provided to second arrayed waveguide grating (AWG 2) 250. Coherent receivers
255 receives that output of AWG 1 240 and AWG 2 250.
[0041] FIG. 3 shows a method of receiving 300 according to examples of the present disclosure.
Method of receiving 300 comprises establishing RF receiver instantaneous RF bandwidth
(IB) requirements at 305. Method of receiving 300 further comprises selecting a first
optical frequency comb (OFC1) with adjacent frequency separation of more than 2xlB
at 310. The number of comb frequencies are two times the number of receiver phased
array elements. In order to increase system gain, OFC1 output power can be maximized.
Method of receiving 300 further comprises selecting a second optical frequency comb
(OFC2) with adjacent frequency as OFC1 + Δ, where Δ is equal to the first channel
bandwidth of the in-phase (I) and quadrature (Q) components provided for DSP at 315.
For example, typical range for Δ is between 10 MHz to 1 GHz. Method of receiving 300
further comprises injecting light from OFC1 into wavelength division multiplexing
(WDM1) element, which has a channel-to-channel separation exactly matching frequency
separation of OFC1 at 320. Each fiber output of WDM1 carries a single wavelength from
OFC1, which is steered towards an electro-optical modulator having RF signal from
phased array element as data input. The modulator can be directly connected to an
antenna element or be at the output of a low noise amplifier (LNA). Once all optical
wavelengths are modulated with the incoming RF field in each modulator, method of
receiving 300 continues by propagating separately in a fiber to a second WDM2 element,
which combines them into a single fiber at 325. The individual fiber paths between
WDM1 and WDM2 is identical to within less than 1mm in order to minimize phase errors
in the following RF beam forming stage. The combined optical wavelengths contain the
total RF field incident on the phased array from all directions. The signal is optically
amplified, which provides uniform RF gain to the total RF field. Method of receiving
300 continues by, in order to separate the individual RF beams incident on the phased
array, splitting the amplified optical signal into N different copies using a wavelength
independent optical splitter at 330. Each output coupled into a fiber has identical
copy of the signal as in the output of WDM2 and it is provided to the N input ports
of an optical non-blocking switch. The lengths of optical paths between the optical
splitter and switch are identical to less than 1mm.
[0042] Optical switch 160 has N input and M outputs, which N is less than or equal to M.
Optical switch 160 is electronically controlled and each input can be steered to any
output. Each output of optical switch 160 is connected to FBG 165, which are designed
with different dispersive properties leading to back reflection of the different wavelengths
from different points along the optical fiber. All wavelengths imported into the switch
are eventually sequentially retroreflected and depending on the dispersion properties
of the FBG different time delay is imposed between them. This process constitutes
true time delay with respect to the RF signals and does not depend on the properties
of the incident RF field. The process of introducing time delays between the different
wavelengths allows in each optical path after the switch to alight the RF fields of
one beam direction.
[0043] FIG. 4 shows a method of signal channelizing 400 according to examples of the present
disclosure. Method of signal channelizing 400 comprises time delaying the wavelengths
in one fiber to align with the phase front of a specific beam incident on the phased
array the information carried by the RF at 405. Channelization provides for smaller
bandwidths for processing and decoding. The channelization uses the fact that each
fiber carries multiple wavelengths each with a replica of RF field. Each fiber from
the output of a circulator is coupled to a WDM3, which separates the different wavelengths
into individual fibers. Method of signal channelizing 400 continues by providing a
second OFC2 with a frequency offset with respect to OFC1 as a local oscillator in
order to channelize the signals at 410. OFC2 is also coupled into a WDM4 in order
to separate the individual wavelengths into separate fibers. An array of optical coherent
receivers equal to the number of wavelengths in OFC1 and OFC2 is used to channelize
the incoming data. Method 400 continues by providing one fiber from WDM3 (e.g., AWG
1 240) and WDM4 (e.g., AWG 2 250) to each optical coherent receiver serving as data
and local oscillator signals in order to extract the in-phase (I) and quadrature (Q)
signal components at 415. This generates N pairs of I and Q signals streams, which
are fed for following DSP analysis. Receiver 100 has N beam forming paths and each
is outputted in an array of coherent receivers. Therefore, receiver 100 generates
total of NxN pairs of I and Q signals representing N RF beams incident onto the phased
array with sub-channel bandwidth equal to IBN.
[0044] FIG. 5 shows a method of beam scanning and signal acquisition 500 used in a search
pattern scenario according to examples of the present disclosure. Method of beam scanning
and signal acquisition 500 can be implemented with a beam former with an optical switch
and FBGs arranged with M-L fixed dispersion FBGs and L tunable dispersion FBFs. Method
of beam scanning and signal acquisition 500 comprises performing a first rough signal
scan search by simultaneously acquiring signals with M-L channels at 505. Once a signal
is identified, method of beam scanning and signal acquisition 500 continues by selecting
a true time delay for the channel with the maximum signal strength and encoding on
one of the tunable FBGs at 510. Method of beam scanning and signal acquisition 500
continues by tuning the tunable FBG dispersion in order to maximize the detected RF
signal at 515. The maximum signal corresponds to optimal alignment with the incoming
RF beam. In some examples, the signal acquisition stability can be improved with a
feedback locking loop. Method of beam scanning and signal acquisition 500 can be repeated
for multiple beams and they can be tracked. Once a beam walks off the range of a tunable
FBG, it can be passed onto a different tunable FBG.
[0045] FIG. 6 shows transmitter 600 according to examples of the present disclosure. Transmitter
600 comprises a transmitter optical frequency comb (OFC) 605 that is configured to
generate a multitude of equidistantly spaced optical wavelengths, as shown in transmitter
spectrum 610. Transmitter 600 also comprises an electro-optic modulator (EOM) 615
that is configured to receive the multitude of equidistantly spaced optical wavelengths
from OFC 605 and data signal 620 and produce a modulated optical beam 625 that is
modulated with data signal 620. In some examples, EOM 615 is a Mach Zehnder modulator.
Other suitable EOMs can also be used. Transmitter 600 further comprises optical circulator
630 that is configured to receive modulated optical beam 625 at an input port.
[0046] Transmitter 600 still further comprises an N input ports by M output ports optical
switch 635 that is configured to receive modulated optical beam 625 from a first output
port of optical circulator 630. Optical switch 635 is further configured to switch
modulated optical beam 625 to an output port of the optical switch terminated in one
or more Fiber-Bragg gratings (FBGs) 640, each FBG having different wavelength dispersion
properties, such that different time delays are introduced between different wavelengths
in each fiber.
[0047] Transmitter 600 still further comprises 1 by P wavelength division multiplexer (WDM1)
645 that is configured to receive individual wavelengths of modulated optical beam
625 that are time-delayed from a second output port of optical circulator 630.
[0048] Transmitter 600 still further comprises plurality of antenna elements 650. Each of
the plurality of antenna elements 650 are configured to receive and transmit an individual
wavelength of time-delayed modulated optical beam. Each of plurality of antenna elements
650 comprise photodiode 665 and power amplifier 660.
[0049] In summary, receiver 100 and transmitter 600 as described allow for receiving, transmitting,
and electro-optically processing multiple simultaneous incoming beams to a phased
array antenna, which eliminates or reduces beam squint. Receiver 100 provides for
first OFC 1125 with each wavelength separated and serving as a separate optical carrier
wave for an optical modulator at each phased array element, with the RF delayed phased
array input modulating each optical modulator. Receiver 100 also provides for N X
M cross-connect optical switch 160 which switches the recombined (but delayed) optical
carrier into different FBGs each tuned to reflect true time delay back into optical
switch 160. Receiver 100 provides for second OFC 2 170 that is coherent but offset
from the first OFC 1125to mix with and demodulate the true time delayed signals to
produce demodulated signals to signal channelization and processing elements 190.
DSP 195 can look at each of the true time delayed signals and determine which one
is from which direction and the best demodulated signal.
[0050] Receiver 100 and transmitter 600 provide for true time domain beam forming, which
allows for processing of signals with tenths of GHz of bandwidth. The use of coherent,
ultra low phase noise optical frequency combs, e.g., OFC 1125, OFC 2 170, OFC 605,
allows for parallel spectral slicing and decomposition of multiple GHz to sub channels
ranging 10 to 100MHz, as well as in applications in RF frequency range 1-100 GHz,
with upper limit set by current practical electro-optical modulators. For example,
receiver 100 allows for receiving the full Ka band 26 to 40GHz, allows for scaling
both in the number of beams and bandwidth at the same time, without suffering from
beam squint, eliminates or reduces the requirement of high ENOB (Effective Number
of Bits) high bandwidth Analog to Digital Converters.
[0051] The advantages of the disclosed receiver and transmitter are achieved, at least in
part, on advancements in coherent optical frequency combs, which provides for ultra-stable
low phase noise microwave sources, time metrology, RF beam forming, signal sampling
and up/down-conversion, frequency channelization and parallel signal processing. Since
photonic frequencies are orders of magnitude higher than typical RF bands, they offer
terahertz of bandwidth, which now with the help of optical frequency combs can be
channelized and sliced to match hundreds RF frequency sub-bands allowing parallel
processing and channelization. Within the V band, a test case signal was analyzed
in the range of 45 to 50 GHz. Microwave photonic implementation of the disclosed receiver
and transmitter modules featuring beam forming, down-conversion and channelization
of a full 5 GHz bandwidth segment to sub 1 GHz selectable sub-channels was achieved.
By leveraging tents to hundreds of coherent optical/frequency combs, simultaneous
readout of antenna elements and then RF beam reconstruction was shown and carried
with predefined dispersive elements setting true time delay, characteristic for the
beam direction. The disclosed features can be applied for multiple beams at the same
time and is data, modulation format, or RF frequency independent, because the true
time delay is implemented in the optical domain. Signals are optically down converted
from RF to base band and channelized to few hundreds of MHz bandwidth with I and Q
streams generated in electrical domain. The disclosed features use electronic digital
signal processing core, wrapped with a photonic layer handling the high frequencies
and beam forming.
[0052] Further, the disclosure comprises embodiments according to the following clauses:
Clause 1: A receiver comprising:
a phased array antenna having P receiving elements configured such that each of the
P receiving elements modulates one of P optical modulators;
a first optical frequency comb configured to generate a multitude of equidistantly
spaced optical wavelengths;
a first 1 by P wavelength division multiplexer (WDM1) configured to receive the multitude
of equidistantly spaced optical wavelengths and further configured to split the multitude
of equidistantly spaced optical wavelengths into individual optical wavelengths such
that each individual optical wavelength is outputted as a separate carrier signal
to each of the P optical modulators such that a total incoming radio frequency (RF)
beam field incident on each of the P receiving elements modulates a separate optical
wavelength in each optical modulator;
a second wavelength division multiplexer (WDM2) configured to receive each output
from each of the optical modulators, such that all modulated wavelengths are combined
together into a single output to form a combined signal; and
an N input ports by M output ports optical switch configured to receive the combined
signal from the WDM2 into at least one input port, and further configured to switch
the combined signal to an output port terminated in one Fiber-Bragg grating (FBG),
each FBG having different wavelength dispersion properties, such that different time
delays are introduced between different wavelengths in each fiber.
Clause 2: The receiver of clause 1, wherein the first optical frequency comb is configured
such that adjacent frequency separation is more than 2 times larger than a maximum
RF frequency expected to be received by phased array receiving elements.
Clause 3: The receiver of clauses 1 or 2, wherein the first wavelength division multiplexer
(WDM1) has channel to channel separation equal to an adjacent frequency spacing of
the first optical frequency comb.
Clause 4: The receiver of any of the clauses 1-3, wherein the second wavelength division
multiplexer (WDM2) has channel to channel separation equal to an adjacent frequency
spacing of the first frequency comb.
Clause 5: The receiver of any of the clauses 1-4, wherein the second wavelength division
multiplexer (WDM2) is configured to have a band-pass more than 2 times larger than
a maximum RF frequency expected to be received by the phased array elements.
Clause 6: The receiver of any of the clauses 1-5, wherein a number of channels in
WDM1 and WDM2 are at least a number of phased array receiving elements.
Clause 7: The receiver of any of the clauses 1-6, wherein optical paths between WDM1
and WDM2, including each optical modulator are equal within 1mm difference.
Clause 8: The receiver of any of the clauses 1-7, further comprising: an optical amplifier;
and
a wavelength independent splitter with N output ports,
wherein the optical amplifier and the wavelength independent splitter are interposed
between the WDM2 and the optical switch such that the WDM2 outputs to the optical
amplifier which in turn outputs to the wavelength independent optical splitter which
in turn outputs to each input port of the optical switch.
Clause 9: The receiver of any of the clauses 1-8, wherein the FBGs at outputs of the
optical switch introduce time delays between the different wavelengths that are representative
of true time delays with which phased array receiver elements are illuminated from
RF beam field wave-fronts arriving from different directions.
Clause 10: The receiver of any of the clauses 1-9, wherein a number of N inputs for
the optical switch corresponds to a number of RF beams that can be simultaneously
acquired on the phased array receiver elements.
Clause 11: The receiver of any of the clauses 1-10, wherein a number of M outputs
from the optical switch correspond to a number of different RF beam directions that
can be resolved.
Clause 12: The receiver of any of the clauses 1-11, wherein the optical switch enables
selection of the output port with the FBG that introduces specific true time delays
between wavelengths representative of a selected RF beam direction.
Clause 13: The receiver of any of the clauses 1-12, further comprising an optical
circulator that is interposed between the WDM2 and the FBGs.
Clause 14: The receiver of any of the clauses 1-13, wherein an output of the optical
circulator carries all wavelengths representing signals from individual phased array
elements forming a specific beam direction due to a true time delay imposed between
different wavelengths.
Clause 15: The receiver of any of the clauses 1-14, wherein the output of the optical
circulator is routed a signal channelization and processing elements.
Clause 16: The receiver of any of the clauses 1-15, wherein simultaneous parallel
acquisition processing of N RF beam directions occurs.
Clause 17: The receiver of any of the clauses 1-16, wherein true time delay formed
RF beams are amplitude balanced using digital signal processing.
Clause 18: A transmitter comprising:
an optical frequency comb configured to generate a multitude of equidistantly spaced
optical wavelengths; an electro-optic modulator (EOM) configured to receive the multitude
of equidistantly spaced optical wavelengths and a data signal and produce a modulated
optical beam that is modulated with the data signal;
an optical circulator configured to receive the modulated optical beam at an input
port; an N input ports by M output ports optical switch configured to receive the
modulated optical beam from a first output port of the optical circulator, and further
configured to switch the modulated optical beam to an output port of the optical switch
terminated in one or more Fiber-Bragg gratings (FBGs), each FBG having different wavelength
dispersion properties, such that different time delays are introduced between different
wavelengths in each fiber; a 1 by P wavelength division multiplexer (WDM1) configured
to receive individual wavelengths of the modulated optical beam that are time-delayed
from a second output port of the optical circulator; and
a plurality of antenna elements, wherein each of the plurality of antenna elements
configured to receive and transmit an individual wavelength of the modulated optical
beam.
Clause 19: The transmitter of clause 18, wherein each of the plurality of antenna
elements comprise a photodiode and a power amplifier.
Clause 20: The transmitter of clause 18 or 19, wherein the EOM is a Mach Zehnder modulator.
While the teachings have been described with reference to examples of the implementations
thereof, those skilled in the art will be able to make various modifications to the
described implementations without departing from the true spirit and scope. The terms
and descriptions used herein are set forth by way of illustration only and are not
meant as limitations. In particular, although the processes have been described by
examples, the stages of the processes can be performed in a different order than illustrated
or simultaneously. Furthermore, to the extent that the terms "including", "includes",
"having", "has", "with", or variants thereof are used in the detailed description,
such terms are intended to be inclusive in a manner similar to the term "comprising."
As used herein, the terms "one or more of" and "at least one of" with respect to a
listing of items such as, for example, A and B, means A alone, B alone, or A and B.
Further, unless specified otherwise, the term "set" should be interpreted as "one
or more." Also, the term "couple" or "couples" is intended to mean either an indirect
or direct connection. Thus, if a first device couples to a second device, that connection
can be through a direct connection, or through an indirect connection via other devices,
components, and connections.
[0053] Those skilled in the art will be able to make various modifications to the described
examples without departing from the true spirit and scope. The terms and descriptions
used herein are set forth by way of illustration only and are not meant as limitations.
In particular, although the method has been described by examples, the steps of the
method can be performed in a different order than illustrated or simultaneously. Those
skilled in the art will recognize that these and other variations are possible within
the spirit and scope as defined in the following claims and their equivalents.
[0054] The foregoing description of the disclosure, along with its associated examples,
has been presented for purposes of illustration only. It is not exhaustive and does
not limit the disclosure to the precise form disclosed. Those skilled in the art will
appreciate from the foregoing description that modifications and variations are possible
in light of the above teachings or may be acquired from practicing the disclosure.
For example, the steps described need not be performed in the same sequence discussed
or with the same degree of separation. Likewise various steps may be omitted, repeated,
or combined, as necessary, to achieve the same or similar objectives. Similarly, the
systems described need not necessarily include all parts described in the examples,
and may also include other parts not describe in the examples. Accordingly, the disclosure
is not limited to the above-described examples, but instead is defined by the appended
claims in light of their full scope of equivalents.
[0055] Notwithstanding that the numerical ranges and parameters setting forth the broad
scope of the present teachings are approximations, the numerical values set forth
in the specific examples are reported as precisely as possible. Any numerical value,
however, inherently contains certain errors necessarily resulting from the standard
deviation found in their respective testing measurements. Moreover, all ranges disclosed
herein are to be understood to encompass any and all sub-ranges subsumed therein.
For example, a range of "less than 10" can include any and all sub-ranges between
(and including) the minimum value of zero and the maximum value of 10, that is, any
and all sub-ranges having a minimum value of equal to or greater than zero and a maximum
value of equal to or less than 10, e.g., 1 to 5. In certain cases, the numerical values
as stated for the parameter can take on negative values. In this case, the example
value of range stated as "less than 10" can assume negative values, e.g. -1, -2, -3,
-10, -20, -30, etc. While the present teachings have been illustrated with respect
to one or more implementations, alterations and/or modifications can be made to the
illustrated examples without departing from the spirit and scope of the appended claims.
For example, it will be appreciated that while the process is described as a series
of acts or events, the present teachings are not limited by the ordering of such acts
or events. Some acts may occur in different orders and/or concurrently with other
acts or events apart from those described herein. Also, not all process stages may
be required to implement a methodology in accordance with one or more aspects or implementations
of the present teachings. It will be appreciated that structural components and/or
processing stages can be added or existing structural components and/or processing
stages can be removed or modified. Further, one or more of the acts depicted herein
may be carried out in one or more separate acts and/or phases. Furthermore, to the
extent that the terms "including," "includes," "having," "has," "with," or variants
thereof are used in either the detailed description and the claims, such terms are
intended to be inclusive in a manner similar to the term "comprising." The term "at
least one of" is used to mean one or more of the listed items can be selected. As
used herein, the term "one or more of" with respect to a listing of items such as,
for example, A and B, means A alone, B alone, or A and B. The term "about" indicates
that the value listed may be somewhat altered, as long as the alteration does not
result in nonconformance of the process or structure to the illustrated implementation.
Finally, "exemplary" indicates the description is used as an example, rather than
implying that it is an ideal. Other implementations of the present teachings will
be apparent to those skilled in the art from consideration of the specification and
practice of the disclosure herein. It is intended that the specification and examples
be considered as exemplary only, with a true scope and spirit of the present teachings
being indicated by the following claims.
1. A receiver (100) comprising:
a phased array antenna having P receiving elements (105), configured such that each
of the P receiving elements (105) modulates one of P optical modulators (120);
a first optical frequency comb (125) configured to generate a multitude of equidistantly
spaced optical wavelengths;
a first 1 by P wavelength division multiplexer, WDM, (130) configured to receive the
multitude of equidistantly spaced optical wavelengths and further configured to split
the multitude of equidistantly spaced optical wavelengths into individual optical
wavelengths such that each individual optical wavelength is outputted as a separate
carrier signal to each of the P optical modulators (120) such that a total incoming
radio frequency, RF, beam field incident on each of the P receiving elements (105)modulates
a separate optical wavelength in each optical modulator (120);
a second wavelength division multiplexer (135) configured to receive each output from
each of the optical modulators (120), such that all modulated wavelengths are combined
together into a single output to form a combined signal; and
an N input ports by M output ports optical switch (160) configured to receive the
combined signal from the second wavelength division multiplexer (135) into at least
one input port, and
further configured to switch the combined signal to an output port terminated in one
Fiber-Bragg grating, FBG, (165), each FBG (165) having different wavelength dispersion
properties,
such that different time delays are introduced between different wavelengths in each
fiber.
2. The receiver (100) of claim 1, wherein the first optical frequency comb (125) is configured
such that adjacent frequency separation is more than 2 times larger than a maximum
RF frequency expected to be received by said phased array receiving elements (105).
3. The receiver (100) of claims 1 or 2, wherein the first wavelength division multiplexer
(130) has channel to channel separation equal to an adjacent frequency spacing of
the first optical frequency comb (125).
4. The receiver (100) of any one of claims 1-3, wherein the second wavelength division
multiplexer (135) has channel to channel separation equal to an adjacent frequency
spacing of the first optical frequency comb (125).
5. The receiver (100) of any one of claims 1-4, wherein the second wavelength division
multiplexer (135) is configured to have a band-pass more than 2 times larger than
a maximum RF frequency expected to be received by said phased array receiving elements
(105).
6. The receiver (100) of any one of claims 1-5, wherein a number of channels in the first
wavelength division multiplexer (130) and the second wavelength division multiplexer
(135) are at least a number of phased array receiving elements (105).
7. The receiver (100) of any one of claims 1-6, wherein optical paths between the first
wavelength division multiplexer (130) and the second wavelength division multiplexer
(135), including each optical modulator (120), are equal within 1mm difference.
8. The receiver (100) of any one of claims 1-7, further comprising:
a first optical amplifier (140); and
a first wavelength independent optical splitter (145) with N output ports,
wherein the optical amplifier (140) and the wavelength independent optical splitter
(145) are interposed between the second wavelength division multiplexer (135) and
the optical switch (160) such that the second wavelength division multiplexer (135)
outputs to the first optical amplifier (140) which in turn outputs to the first wavelength
independent optical splitter (145) which in turn outputs to each input port of the
optical switch (160).
9. The receiver (100) of claim 8, wherein the FBGs (165) at outputs of the optical switch
(160,) are configured to introduce time delays between the different wavelengths that
are representative of true time delays with which phased array receiving elements
(105) are illuminated from RF beam field wave-fronts arriving from different directions.
10. The receiver (100) of claims 8 or 9, wherein a number of N inputs for the optical
switch (160) corresponds to a number of RF beams that can be simultaneously acquired
on the phased array receiving elements (105).
11. The receiver (100) of any one of claims 8-10, wherein a number of M outputs from the
optical switch (160) correspond to a number of different RF beam directions that can
be resolved.
12. The receiver (100) of any one of claims 8-11, wherein the optical switch (160) is
configured to enable selection of the output port with the FBG (165) that introduces
specific true time delays between wavelengths representative of a selected RF beam
direction.
13. The receiver (100) of any one of claims 8-12, further comprising a first optical circulator
(150) that is interposed between the second wavelength division multiplexer (135)
and the FBGs (165).
14. The receiver (100) of claim 13, wherein an output of the first optical circulator
(150) carries all wavelengths representing signals from individual phased array elements
forming a specific beam direction due to a true time delay imposed between different
wavelengths and furthermore, wherein the output of the first optical circulator (150)
is routed to a signal channelization and processing elements (190).
15. The receiver (100) of any one of claims 1-14, wherein true time delay formed RF beams
are amplitude balanced using digital signal processing.